I am trying to learn how to use an o'scope.

[Hextejas]

I preparation to building some audio gear.  I have an el cheapo scope, a DOS 138. I figured that it was good enuf for training for awhile.
This is what I am doing. Part of the DSO is a 1kHz, 3.3v generator. So I have that hooked up and the screen is displaying the proper waveform.
Next, I installed 2 waveform generators on my Android 10.5", notebook.
They are: Kewlsoft and Frequency Sound Generator.
I dialed them both to 1 kHz, the voltage, I don't know. I also selected a square waveform.
But listening to them by headphone, they sound the same.
Then, I took a spare stereo headphone cable, cut the jack off, and bared the wires. There were a red, black, and bare silver.
I had to strip the insulation from the red and black. The silver was already bare.
I attached the scope probe to the read headphone lead and expected to see the same thing. Nope, and it's hard to describe.
Almost flat with an occasional pulse. When I have the scope display the numbers, it says 60hz.
Scope settings were the same, .5ms and 1v.

So, why does the scope not produce the same wave form ?

Thanks

[rs20]

Where did you connect the ground lead of the scope? It should be connected to the bare silver shielding braid, not to the black wire (bare silver shielding is ground, black and red are the Left and Right audio channels).

[paulca]

You will need to select AC coupling on the scope to view the waveforms properly.

With the scope on AC you can dial the voltage down to about 200mV (per division).  1mS time base or smaller.

The Audio output jack on a phone/tablet is roughly around 500mV-1V peak to peak.

Check the "trigger " setting is not set to "ONCE", AUTO or NORM are probably better.  If the frequency is not stable is is often better to hit HOLD or PAUSE yourself to see what is going on.

[rs20]

You will need to select AC coupling on the scope to view the waveforms properly.

Eh? The waveforms in coming out of the headphone jack of the phone should already be AC-coupled, i.e., completely free of any DC offset. So whether AC or DC coupling is selected on the oscilloscope should be irrelevant.

But don't let me confuse you OP, there's certainly no harm in choosing AC coupling as recommended by paulca.

[paulca]

You will need to select AC coupling on the scope to view the waveforms properly.

Eh? The waveforms in coming out of the headphone jack of the phone should already be AC-coupled, i.e., completely free of any DC offset. So whether AC or DC coupling is selected on the oscilloscope should be irrelevant.

But don't let me confuse you OP, there's certainly no harm in choosing AC coupling as recommended by paulca.

Agreed.  It probably won't matter.  However it's a good habit to get into, trying to read a waveform on a signal with a 5V virtual ground would be quite tricky in DC mode.

[Brumby]

The DSO138 is a bit of a toy (I have one) but it should be useful enough for this sort of audio work.

The one thing I found with it, though, is that the user interface isn't particularly pleasant to deal with.  It doesn't have a "beam finder" option and you need to become familiar with how the ranges work, before you start hunting signals.

The other thing is that you need to have the scope connected at two points - to display the voltage between them - and you haven't been clear as to what you have connected the scope to.

If you have put one scope lead on the black and the other on the red, you will be measuring the difference between the left and right channels.  For mono signals, the left and the right will be the same - so you are not going to get ANY difference signal to measure, except perhaps for the odd occasional bit of noise.

As mentioned above, you should connect the scope "ground" to the bare silver wire and the scope signal lead to either the black or the red.  The bare silver wire will be the common and the red and the black will be the left and right channels (or the right and the left).

[Hextejas]

Ok, that helped a lot though the square wave looks a bit rough.
Now that I know what 1khz is supposed to look like, I can pump it into my breadboard amp and see what comes out the other side.
Maybe y'all can critique the schematic ( I hope )

[Brumby]

Yes, you have some healthy overshoot and ringing there.

If you have no load on the circuit, that can cause funny things.  Just as an experiment, try putting a resistor across the scope input, while it is still connected to the signal.  Start with, say, a 1K, then try a 470R, 220R, 100R, 47R and maybe a 22R and see how the waveform changes.  (These values aren't particularly critical - you get the idea.  Just don't go too low)

I expect the ringing to decrease with lower resistance - but you don't want to go too low as it might overload the output stage of your signal source.  If you can find out the minimum load impedance specification for your tablet, that will tell you how low you can go.

If it does what I expect, then you will have learned something.  If it doesn't , then you will have learned two things.

[Hextejas]

Thank you Brumby I will try that next.
As I started trying to figure out where to put the probes on my breadboard, I came to an impasse.
That is signal gnd vs power gnd.
Coming from the IPad, the silver wire is signal gnd, whereas, I also have a gnd (-15v DC), coming from the power supply.
Hmmm, there will also be a gnd as part of the audio speaker.
It sure don't seem correct to wrap those all together.

Do you think that I should start another thread as I begin to use this scope with my nascent amplifier?

And thanks a lot.

George

[rstofer]

Might as well stick with this thread as long as the oscilloscope connection is the primary concern.

We really can't say anything about how to probe your amplifier without a schematic.  I would ASSUME that the incoming shield sets up 'ground' for the entire amplifier but I don't really know that.

When the amplifier is the main topic, perhaps another thread would be helpful.

Either way...

[Hextejas]

Re connecting the other probe to the silver wire, I did not know what to do with the other scope probe and I sorta assumed that I did not need it to be connected, because with the scope generated test square wave, I didn't seem to need it. The wave form was nice and pretty without it being connected.

And embarrassingly I admit to not knowing what an incoming shield is. 

[Brumby]

If you are using the built-in square wave generator of the DSO138, then you won't need to connect the external common/earth/ground connection ... it's already connected internally.

You will have to pay more attention when you are connecting separate pieces of your setup.

[rstofer]

And embarrassingly I admit to not knowing what an incoming shield is. 

I was talking about the shield on the audio cable in the original post.  There were supposedly 2 channels of output on the black and red wires and the shield for common (and 'ground').  I thought we had gotten to the point where you had a waveform when you connected the scope ground to the shield and the probe to one of the colored wires.

[Hextejas]

And embarrassingly I admit to not knowing what an incoming shield is. 

I was talking about the shield on the audio cable in the original post.  There were supposedly 2 channels of output on the black and red wires and the shield for common (and 'ground').  I thought we had gotten to the point where you had a waveform when you connected the scope ground to the shield and the probe to one of the colored wires.

We had, rstoffer, all is well. I did not know that the skinny strand, bare wire was a shield. At most it was 3 wires running alongside the red and black.

[james_s]

-15V is not normally "ground" but rather the negative supply rail. I'm assuming you have a bipolar supply, for example + and - 15V, and then you will have a common ground in the middle. That's where you want to ground the scope, normally that's the signal ground as well.

[paulca]

We had, rstoffer, all is well. I did not know that the skinny strand, bare wire was a shield. At most it was 3 wires running alongside the red and black.

Are you trying to use an old headphone lead?  They have terribly thin conductors and are often just coated bare wire.  Not easy to work with.

A good cable... even a cheapy should have a good chunk of shield wire.  The bonded pair cables which are common have a common/ground in each side, one with Right, one with left which can be twisted together.

A good solid ground connection with these wires is very important.  With they floating around or badly connected you will get barely any signal and a ton of interference and usually mains hum.

[Hextejas]

It seems that I need a better signal wire as you suggested  Paulca.
The signal has been cleaned up some but is still very jittery and I swear that, as I was holding a resistor in place, the wave was throbbing in sync with my heartbeat. Weird.
I tried a few resistors starting with 1k and also tried wiring it differently. By across the leads I assume it means connect the red to the silver. That had no effect, however when I connected the resistor in series with the red lead it made quite a difference though I had to jack up the rating to 330k.
I don't really know what it did to the signal in terms of the audio effect.  I will save that experiment for another day. I sure wish that I had a " dial the resistor " device.

[Brumby]

Sometimes providing a photo makes things a bit easier for us to understand exactly how you have set something up - not just for showing us the scope display.

Since you have pretty decent photographic skills    I would suggest putting up a photo every now and then - so we can see what is connected to what.  A picture is worth a thousand words, as they say.

[Hextejas]

Thanks for all the help and I think that now i am ready to take my new found scope skills forward into the amp.
My intent is to post the schematic and breadboard it a piece at a time. Starting with a better signal-in device set at 1kHz, and take measurements a step at a time.
I have all the pieces and tried slamming it all together but I am sure that I got my wires crossed, so its back to step 1.
Paulca, would you happen to knoiw the name of a better headphone out connector ?
I don't think that I have ever heard of one.

[rs20]

Paulca, would you happen to knoiw the name of a better headphone out connector ?
I don't think that I have ever heard of one.

Paulca did not say use a better connector, he said use a better cable. There's no problem with 3.5mm connectors in general. But there can be a great variation in the quality of the cables.

[Brumby]

But there can be a great variation in the quality of the cables.

This goes to 2 things - how cheap the manufacturer wants to go, but -more importantly - that the signal travelling down these wires on the way to headphones, etc. is comparatively large.  It has to drive a mechanical transducer and this requires more power than a signal going from a device to an amplifier.  As a result, it doesn't need as much in the way of shielding.

Using such a cable with lower level signals could cause you some distress.  Without decent shielding, picking up extraneous noise will be so much easier - and eliminating it so much harder.

[paulca]

Your scope is showing 20mVpp.  If that is correct and not an artefact of a 10x probe or your resistor then you are facing problems on any cheap home setup.

In my lab I have around a 50-70mVpp noise floor.  Meaning that no matter what I measure, if I zoom the scope in to 50mV per division I can see white/pink noise with a max peak to peak of 50-70mV.  Of course it's average is much less but the scope usually reads somewhere between 50mV and 70mV.

(I was surprised to see my solar panel monitor also registering 0.050V panel voltage yesterday when the panel was disconnected leaving the input "floating", but it just confirmed my lab has 50mV of noise, no matter what I do.)

Working with 20mVpp signals will require very good shielded cables and a electrically quiet set up.  None of which you will get with that scope or a cheap headphone cable.

A signal generator, even a 5.99 chinese kit one like this:
https://www.ebay.co.uk/itm/XR2206-Function-Signal-Generator-Sine-Triangle-Square-Wave-1HZ-1MHZ-DIY-Kit-HW/253335835050?_trkparms=aid%3D555019%26algo%3DPL.BANDIT%26ao%3D1%26asc%3D43782%26meid%3Dea6faf48fbd44623a50fd947ddda7244%26pid%3D100507%26rk%3D1%26rkt%3D1%26%26itm%3D253335835050&_trksid=p2045573.c100507.m3226

Will produce 1Vpp or more.  In fact on square wave (which can't be attenuated on the linked signal gen) is around 10V peak to peak.  Which is fairly useless for testing audio electronics.

A good line level output from say a phone is usually between 500mVpp and 1Vpp.

On a budget, why not try something like this:
https://play.google.com/store/apps/details?id=com.keuwl.functiongenerator&hl=en_GB

On cables, these style tend to be better as they have separate insulated wires for left and right.  So you get double the conductor for ground/shield.
https://www.ebay.co.uk/itm/1m-Right-Angle-3-5mm-Stereo-Jack-to-2-RCA-Phono-Plugs-Cable-Gold-007915/191767603264?epid=1453132509&hash=item2ca63d6840:g:a34AAOSw3xVaHfCa

[Hextejas]

I actually have that generator but was using a different one. I didn't know that I would need to crank up the voltage so I will do it. 1 v you say ?
Re the cable, I have a similar set as shown though I don't knoiw where I would connect the black wire from the scope. Oh wait, its the outside of the connector, right ?

[paulca]

500mV peak to peak, so +250mV to -250mV, will give you a "representative" line level signal, some output more, some less.  Note that microphones or turntables would be much lower.

With regards the lead, you "can" crocodile clip onto the centre pin of each RCA/Phono plug for Left Signal, Right Signal.  The black wire (ground clip) would go to ideally both outer rings, you can probably get away with one though.

The downside is this gets fiddily.  The croc clips slip off the center pin, or short against the outer ring.  You'll end up cursing and shorting out your phone.  (It will probably survive it fine though)

Best option is to buy some of these:
https://www.ebay.com/itm/TRRS-3-5mm-Jack-Breakout-Headphone-Video-Audio-MP3-Jack-Professional/351577851100?hash=item51dbac44dc:g:UQQAAOSw4UtWRX0m

and associated jack->jack leads.

The breakout boards are TRRS (Tip, Ring, Ring, Sheild).  Audio connectors are TRS (Tip, Ring, Sheild), but you can just wire the Ring and shield together.

Sheild = GND
Ring2 = GND
Ring1 = Left
Tip = Right

[Hextejas]

Ok, parts are on order and with 2 day shipping no less. 
I have no patience at all and want to get working on it right this minute, though I am busy doing the schematic of the amp.
Actually, it's someone elses and I am borrowing it.

Did I remember to say thanks ?
If not, then thanks